This invention relates to the formation of sharp edges on blades by grinding or honing techniques, and more particularly concerns a method and apparatus for sharpening edges of substantially flat metal blades by advancing the blade in contact with rotating members.
In the production of razor blades, a continuous ribbon of flat metal stock, called a blade strip, is advanced in continuous contact with rapidly rotating abrasive wheels which sharpen one or both edges of the blade strip. The sharpening is achieved by removing metal in a continuous facet zone in each face of the blade strip adjacent an edge thereof. the facets are substantially equally but oppositely angled with respect to the face of the blade strip, and converge to an intersection which constitutes the cutting edge. A stropping operation generally follows the grinding step, its purpose being to remove any clinging fragments of metal and to generally smooth out any surface irregularities produced by the grinding step.
One method of making razor blades utilizes a series of disc-shaped grinding wheels stacked in parallel array with intervening spacers on two spindles whose axes of rotation closely parallel the direction of travel of the blade strip. The wheels of one array are intertwined or dovetailed with wheels of the other array in a manner whereby the spacer of one spindle is opposite a wheel of the other spindle. The blade strip travels along the nip or intersection of said interactive wheels in perpendicular disposition thereto, whereby the circular perimeters of the wheels of one array grind one facet, and the other array grind the opposite facet. The reason for the aforesaid arrangement of wheels is to overcome the phenomenon of burr formation, namely the buildup of metal on the side of the blade opposite the facet being ground. Another advantage of the opposing wheels is that support is provided for the blade strip which would otherwise bend away from the abrading wheel.
More recent methods of making razor blades, as disclosed in U.S. Pats. No. 2,281,975 and 2,290,964, replace the two opposed parallel arrays of wheels with two opposed identical wheels having a continuous spiral flute abrading surface disposed in a circular cylindric locus and bounded by a continuous spiral groove. The two wheels are disposed in a manner such that their cylindric axes are parallel, and the abrading flute of one wheel slightly penetrates into the groove of the opposing wheel. Such manner of wheel interengagement is preserved during rotation by gearing the wheel spindles so that they rotate synchronously in opposite directions.
The effect of the interengaged spiral wheels is to alternatively push the burr from side to side. The successive reversal of strain on the burr results in work hardening and ultimate fracture of the burr. Although more expensive, an advantage of the spiral wheel over the parallel array of spaced wheels is that the number of strain reversals on the burr is determined by the RPM of the spiral wheels. A further consequence of this effect is that the blade strip need not be advanced to be sharpened. However, for reasons of economics in the manufacture of razor blades, it is desirable to advance the blade strip at the highest possible speed.
In order to achieve proper sharpening, the blade strip, moving at high velocity past the grinding wheels, must experience an adequate number of strain reversals at the facets. Although faster rotation of spiral wheels can produce increased reversals, the spacing between adjacent turns of the abrasive flute is generally greater than the spacing achievable by the close stacking of flat grinding wheels. For example, in a spiral wheel of three inch cylindrical diameter, a spiral flute of one turn per axial inch would be the maximum practical pitch. By way of comparison, using flat wheels of 1/4 inch thickness (or only a few thousandths smaller to allow clearance) two spaced pairs could be assembled within one inch of axial length, giving four strain reversals, which is twice greater than said spiral wheels, assuming one revolution per inch of blade travel.
It is accordingly an object of the present invention to provide a method for sharpening a blade strip at high linear velocity of said blade strip.
It is another object of the present invention to provide sharpening apparatus for causing a high number of burr reversals within a short distance of axial travel of said blade strip.
It is a further object of the present invention to provide grinding apparatus capable of sharpening flat metal blades without requiring linear motion of said blades past said grinding apparatus.
These objects and other objects and advantages of the invention will be apparent from the following description.